The sounds of the police motorcycles are the sped-up sounds of women screaming together in a tiled bathroom.
George Lucas apparently named the film after his San Francisco telephone number, 849-1138, the letters THX correspond to letters found on the buttons 8, 4, and 9.
To provide the large number of extras required, George Lucas contacted the Synanon drug rehabilitation facility. He found many recovering drug users who were required to be shaved bald for the drug program anyway. The Synanon Facility is mentioned in Philip K. Dick's science fiction novel "Valis".
Very little makeup was used on the actors and actresses, so as to give them a look of authenticity.
George Lucas adapted the idea for the original short, and eventually this movie, from a one and a half page outline called "Breakout", which was written by fellow USC student Matthew Robbins, about a man escaping an underground dystopian society.